Traditional methods for diagnosing infectious diseases rely on microscopic visualization of pathogens in a patient body fluid or tissue sample or the growth of the microorganism in the laboratory with the subsequent identification of the microorganism using clinical diagnostic tests. These techniques are generally reliable but are often time-consuming. Additionally, conventional methods of diagnosis, such as physical examinations, X-ray, CT scans, and ultrasonography, often fail to identify the location and/or the extent of infection in sites of localized infection (e.g., an abscess). Identifying the site of a localized infection and determining the extent of infection is important because rapid localization and rapid determination of the extent of an infection is critical to effective therapeutic intervention. If left untreated, infections can damage host tissue, or can become chronic. Infections can also become systemic by entering the bloodstream leading to sepsis, which is a serious, rapidly progressive, multi-organ disease state. Infections can also become gangrenous, resulting in loss of an infected limb, or even death.
An inflammatory response is one of the first responses to infection. Inflammation results in increased blood flow to the area of infection, release of chemicals (e.g., chemoattractants) to attract leukocytes to the site of infection, and migration of immune cells, such as monocytes and macrophages, to the site of infection as a defense against the invading microorganisms. Inflammation can be initiated by production of eicosanoids and cytokines, which are released by injured or infected cells. Eicosanoids include prostoglandins which promote fever and dilate blood vessels during an inflammatory response, and leukotrienes which attract certain leukocytes to the site of infection, including monocytes and macrophages.
Monocytes and macrophages are vital to the regulation of immune responses and to the development of an inflammatory response. As part of the innate immune system, macrophages and monocytes are continuously monitoring the tissues in which they are located and thus play a major role when bacteria are encountered. Specifically, macrophages have the ability to recognize pathogen-associated molecular patterns through their surface receptors (e.g. toll-like receptors) and help orchestrate the secretion of pro-inflammatory cytokines and other mediators, as well as engage in the direct phagocytosis of the invading microorganism. Common cytokines that are involved in inflammation include interlerkins responsible for communication between white blood cells, and chemokines that promote chemotaxis during an inflammatory response. Cytokines both recruit immune cells to the site of infection and cause immune cell activation during an inflammatory response.
Folate receptors are expressed on a subset of macrophages (i.e., activated macrophages), and on monocytes. Monocytes and macrophages participate in the immune response by nonspecifically engulfing and killing foreign pathogens, by displaying degraded peptides from foreign proteins on their cell surfaces where they can be recognized by other immune cells, and by secreting cytokines and other factors that modulate the function of T and B lymphocytes, resulting in further stimulation of immune responses.
The overexpression of folate receptors on activated macrophages, and on monocytes, is described in U.S. Patent Application Publication No. US-2007-0009434-A1 and U.S. Patent Application Publication No. US-2002-0192157-A1, both of which are incorporated herein by reference. Additionally, compounds and methods for targeting radionuclide-based imaging agents to cells expressing the folate receptor are described in U.S. Pat. No. 7,128,893, incorporated herein by reference. Furthermore, compositions and methods for diagnosing and monitoring, using positron emission tomography, pathogenic disease states wherein the pathogenic cells uniquely express, preferentially express, or overexpress vitamin receptors are described in PCT Publication No. WO 2006/071754, incorporated herein by reference.
In one embodiment, a method is provided of detecting a site of localized infection by imaging a population of inflammatory cells, said method comprising the steps of administering to an animal suffering from an infection an effective amount of a composition comprising a conjugate of the formula Ab-X wherein the group Ab comprises a vitamin and the group X comprises an imaging agent, and detecting the site of localized infection. In this embodiment, group X can further comprise a liposome, the vitamin can be selected from the group consisting of folate, riboflavin, thiamine, vitamin B12, and biotin, the imaging agent can comprise a metal chelating moiety, the imaging agent can further comprise a metal cation, the metal cation can be a radionuclide, the radionuclide can be selected from the group consisting of isotopes of gallium, indium, copper, technetium, and rhenium, the composition can be administered parenterally to the animal, and the imaging method can be selected from the group consisting of computed tomography, positron emission tomography, magnetic resonance imaging, ultrasound, and single photon emission computed tomography, or a combination thereof.
In yet another embodiment an imaging method is provided for detecting localized infections. The method comprises the steps of administering to the patient a composition comprising a conjugate Ab-X which has the formula
wherein V is a vitamin, L is a divalent linker, R is a side chain of an amino acid, M is a cation of a radionuclide, and n is 1, and detecting the site of the localized infection. In this embodiment, V can be a vitamin selected from the group consisting of folate, riboflavin, thiamine, vitamin B12, and biotin, the radionuclide can be selected from the group consisting of isotopes of gallium, indium, copper, technetium, and rhenium, the composition can be administered parenterally to the animal, and the imaging method can be performed by a method selected from single photon emission computed tomography and computed tomography, or a combination thereof.